The was some question where the ship was located when it was sunk. I am not an expert on Maritime Law and when and where a ship can be sunk.
It was war, the Argies started it, and Thatcher finished it.
Under maritime law, the Belgrano could be sunk anywhere. Argentina had committed an act of war against the United Kingdom, and the UK had the right to sink an Argentinian warship anywhere under virtually any circumstances.
Whether or not Belgrano was in the exclusion zone was utterly irrelevant. What the exclusion zone did was set up an area where the United Kingdom said that it might sink any ship in that area, regardless of the flag it was flying - you can't always reliably identify the nationality of a ship. It was a warning to ships of neutral nations to avoid the area so they would remain safe. It had no relevance to a belligerent nation.
The Argentinian Navy, and its sailors, understand this - and have rarely made any fuss about what happened. They were at war, and would have sunk HMS Conqueror if they'd had the chance. It is a tragedy in a real sense - those Argentinian sailors were brave men serving their country, and had no part in their governments decision to invade the Falklands - but in war, brave, innocent men die on both sides. That's the way it works.